I am sure that everyone wonders if their backup time is acceptable when they first use USB 3.0. I am at that stage too after doing my first TM backup of my mini, using USB 3.0 to a USB 3.0 external. I watched the speeds on the activity monitor and they fluctuated so much that it is not a good gauge.

But I did take a screen shot with the time started and time ended. So my very first Time Machine backup of 129 GB took 57 minutes, so I think that could be roughly rounded out to 2 GB a minute.

My answer in short is file size is going to create variance and TM can be slow. I am assuming the drives are both spinners?

I get a little over 200 MB/s for copying media files and large graphics files from a TB RAID (4 spinners, RAID 0) to a single SSD USB3 enclosure. Copy same files from TB RAID to PCI SSD in my MacPro I get sustained 550 MB/s. And finally same files from TB RAID to TB RAID, ~235 MB/s.

2GB per minute is pretty good for my experience with TM backups and the rates I give don't reflect a TM backup